Functional analysis of ADARs in planarians supports a bilaterian ancestral role in suppressing double-stranded RNA-response

ADARs (adenosine deaminases acting on RNA) are known for their adenosine-to-inosine RNA editing activity, and most recently, for their role in preventing aberrant dsRNA-response by activation of dsRNA sensors (i.e., RIG-I-like receptor homologs). However, it is still unclear whether suppressing spurious dsRNA-response represents the ancestral role of ADARs in bilaterians. As a first step to address this question, we identified ADAR1 and ADAR2 homologs in the planarian Schmidtea mediterranea, which is evolutionarily distant from canonical lab models (e.g., flies and nematodes). Our results indicate that knockdown of either planarian adar1 or adar2 by RNA interference (RNAi) resulted in upregulation of dsRNA-response genes, including three planarian rig-I-like receptor (prlr) homologs. Furthermore, independent knockdown of adar1 and adar2 reduced the number of infected cells with a dsRNA virus, suggesting they suppress a bona fide anti-viral dsRNA-response activity. Knockdown of adar1 also resulted in lesion formation and animal lethality, thus attesting to its essentiality. Simultaneous knockdown of adar1 and prlr1 rescued adar1(RNAi)-dependent animal lethality and rescued the dsRNA-response, suggesting that it contributes to the deleterious effect of adar1 knockdown. Finally, we found that ADAR2, but not ADAR1, mediates mRNA editing in planarians, suggesting at least in part non-redundant activities for planarians ADARs. Our results underline the essential role of ADARs in suppressing activation of harmful dsRNA-response in planarians, thus supporting it as their ancestral role in bilaterians. Our work also set the stage to study further and better understand the regulatory mechanisms governing anti-viral dsRNA-responses from an evolutionary standpoint using planarians as a model.